The turbine section of a gas turbine engine contains a stationary shroud ring around each stage or row of rotating blades. A shroud ring has a sealing surface with close clearance to the blade tips to reduce inter-stage gas leakage past the turbine blade tips. The shroud ring is formed of curved segments called ring segments or blade outer air seals. Turbine blades and ring segments are commonly made of high temperature superalloys based on nickel, cobalt, or nickel-iron that maintain mechanical strength, creep resistance, surface stability, and corrosion/oxidation resistance at high temperatures. The sealing surface of each ring segment is typically coated with an oxidation resistant metallic bond coat and a thermally insulating ceramic thermal barrier coating (TBC). The outermost layer of the TBC is generally formed to be porous to make it abradable. When the metal turbine blade tips rub against the abradable coating, the blades cut a swath through the coating, thus enabling a minimal clearance between the blade tip and ring segment to minimize working gas leakage between turbine stages, thereby maximizing power output and fuel efficiency.
Abradable coatings are typically sprayed much thicker than other types of thermal barrier coatings. Extra thickness is required to allow the blade tips to cut into the coating. As thickness increases, strains increase in the coating from thermal gradients, especially during startup and shutdown of the engine. This can increase spalling of the coating causing loss of clearance control, which reduces engine power and efficiency. It also reduces thermal protection of the ring segment, reducing its service life. Another cause for spalling is increased hardness of the abradable coating through gradual sintering during engine operation.
When a thermal barrier coating has become degraded, it must be replaced or repaired to maintain engine efficiency. This is normally done by removing the ring segment to a repair facility, removing all of the old coating, and applying a new coating by known thermal spray methods. This is expensive, energy-intensive, and time-consuming. If the substrate is degraded by surface cracking as a result of the degraded thermal barrier coating, the ring segment may need to be replaced.